1. Field of the Invention
The present invention relates to a color video recorder/reproducer wherein wide bandwidth luminance information is alternately recorded with narrow bandwidth luminance plus chrominance information for the lines of a television picture frame. Upon playback, a video signal is reconstructed by combining the signal recorded for a given line with a complementary portion of the signal recorded for the preceding line.
2. Description of the Prior Art
This application is related to U.S. Patent Application Ser. No. 661,204, filed Feb. 25, 1976 by William Kelsey Hickok and of common assignee. One of the most troubling aspects of color video recording is the relatively large bandwidth needed to record a standard color video signal. Even allowing for acceptable bandwidth attenuation, a minimum bandwidth recording capability of about 4.5 MHz is required. A recording system for handling a signal having information contained in frequencies from about 30 Hz to 4.5 MHz (as in a typical color video signal) is almost unthinkable from an equalization standpoint alone. For example, it is well known that the gap effect causes an increase in head response of about 6 db per octave as frequency increases until a maximum response is reached, at which point the response rapidly drops off to zero. A signal extending from 30 Hz to 4.5 MHz covers over 17 octaves; and a head designed for maximum response at 4.5 MHz will have its response down by more than 100 db at 30 Hz. Such a response range is far too great to be corrected by conventional equalization means.
There are two avenues of approach one may follow to get around the equalization problem. One approach is to frequency shift the video signal to a higher frequency range. The shift can be accomplished using an FM recording system or a direct recording system. In an FM recording system, the video signal may be used to frequency modulate a carrier having a center frequency of about 6 MHz, thus producing sidebands extending from about 1.5 MHz to 10.5 MHz. Such a frequency range represents a range of about three octaves and equalization is easily accomplished. (Generally, equalization may not be necessary in FM recording systems, since amplitude variations, so long as they are not too severe, are completely eliminated in the FM detection process.) In a direct recording system, the video signal can be shifted "as is" to occupy a frequency range from about 1 MHz to 5.5 MHz; again resulting in a range of about three octaves and presenting little problem from an equalization standpoint.
From a cost viewpoint, however, the outlook is not as promising. Both the FM and direct recording systems described above require the handling of a signal containing relatively high frequencies; as high as 5.5 MHz in the direct recording system and 10.5 MHz in the FM recording system. To record high frequencies, it is necessary to provide a high tape-to-head speed. And not only must high frequencies be recorded and reproduced but, importantly, they must be exactingly reproduced. Even seemingly insignificant amounts of time base error may show up as horizontal jitter, bending, or smearing. The cost of apparatus of generate the high tape-to-head speed, and the cost of control systems to ensure the high degree of time base stability which is so crucial, can be impressively high. To provide a video recorder/reproducer suitable for "home" use would seem to require another approach.
The other avenue of approach to the equalization problem is to obviate the necessity for recording high frequencies by narrowing the bandwidth of the recorded signal. For example, if the video signal could be made to occupy a bandwidth of 2 MHz, such a signal could be recorded in a frequency range extending from 0.5 MHz to 2.5 MHz (assuming a direct recording system). Since this frequency range represents only about three octaves, the head response may be equalized in a conventional manner. Further, the highest frequency to be recorded is only 2.5 MHz, thereby enabling a tape-to-head speed of one-half that of the direct recording system and one-fourth that of the FM recording system previously described.
U.S. Pat. No. 3,255,303 discloses a method of reducing the bandwidth of the recorded video signal. The method is based upon the realization that the color video signal, as transmitted, contains more information than is absolutely necessary for an acceptable picture. The patent relates to a television signal transmission system wherein luminance information is alternately recorded with chrominance information for each line of a television picture field. The video signal thus recorded contains only about half the information content of the original video signal and can, therefore, be made to occupy a bandwidth about one-half as wide as the bandwidth of the original video signal. The video signal thus recorded cannot be transmitted to a television receiver without some processing; otherwise, each line of the transmitted field would contain only luminance or chrominance information, never both. The resultant picture would be unacceptable both with respect to monochrome and color picture quality. In an attempt to produce an acceptable picture, a video signal for transmission is reconstructed wherein a given line comprises its recorded luminance (or chrominance) information plus the chrominance (or luminance) information of the preceding line.
A video recorder was "breadboarded" embodying an alternating line recording technique akin to the recording scheme disclosed in U.S. Pat. No. 3,255,303, above. In this breadboard system, luminance information was recorded for the first transmitted line, chrominance information for the next transmitted line, etc. The recorded video signal thus contains luminance information for lines 1, 5, 9 . . . 525 in the odd line field and lines 4, 8, 12 . . . 524 in the even line field. There is a "gap" of luminance information for lines 2 and 3, 6 and 7, etc. If these luminance gaps are not filled in, the reproduced picture display appears to have been stripped of luminance information for every other line pair. To fill in the luminance gaps, a video signal is reconstructed wherein whatever information is recorded for one line is delayed and added to the next line. In this manner, line 6, for example, contains the old luminance information held over from line 4 and line 7 contains the old luminance information held over from line 5. The luminance gaps are therefore filled in with old luminance information (in an analogous manner, chrominance gaps are filled in with old chrominance information). While using old luminance information is better than leaving luminance gaps, luminance information from line 1 (delayed and played back with line 3) is nevertheless juxtaposed with luminance information for line 4, and similarly for lines 5 and 8, 9 and 12, etc., thereby resulting in a picture which, when viewed, displays an irritating amount of vertical aberration. Since using old luminance information is apparently the only way of filling in the luminance gaps, and since using old luminance information makes for an irritating picture display, it seems that a video recorder/reproducer employing the alternating line recording scheme has an inherent incapability of producing a television picture of acceptable quality.